Outside pressure like the wind on the clock hands can affect the clock's accuracy. Edmund Beckett Denison designed the 'Double Three-legged Gravity Escapement' to solve this. It compensates for the effects of outside pressure on the mechanism and is crucial in helping the clock keep accurate time.

The accuracy of the Great Clock of Westminster (Big Ben) is primarily due to its "Double Three-legged Gravity Escapement," an ingenious 1851 invention by Edmund Beckett Denison. This mechanism isolates the pendulum from external forces like wind, snow, or bird interference on the massive clock hands, ensuring the "tick" remains constant. For fine-tuning, the clock uses a quirky but effective Victorian method: pennies. Adding a single pre-decimal penny to a tray on the 13-foot pendulum slightly raises its center of gravity, causing the clock to gain 0.4 seconds per day. Conversely, removing a penny slows it down. This analog system allows the clock keepers to maintain the time to within one second of the actual time, even after more than 160 years of operation and several major structural renovations.